XRT72L56 Datasheet, PDF (315/486 Pages) Exar Corporation – SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER

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PRELIMINARY
XRT72L56
SIX CHANNEL DS3/E3 FRAMER IC WITH HDLC CONTROLLER
REV. P1.1.2
NOTES:
1. The default condition is the Bipolar Mode.
2. This selection also effects the operation of the
Receive E3 LIU Interface block
5.2.5.1.1 The Bipolar Mode Line Codes
If the Framer is choosen to operate in the Bipolar
Mode, then the DS3 data-stream can be choosen to
be transmitted via the AMI (Alternate Mark Inversion)
or the HDB3 Line Codes. The definition of AMI and
HDB3 line codes follow.
5.2.5.1.1.1 The AMI Line Code
AMI or Alternate Mark Inversion, means that consec-
utive "one's" pulses (or marks) will be of opposite po-
larity with respect to each other. The line code in-
FIGURE 123. ILLUSTRATION OF AMI LINE CODE
volves the use of three different amplitude levels: +1,
0, and -1. +1 and -1 amplitude signals are used to
represent one's (or mark) pulses and the "0" ampli-
tude pulses (or the absence of a pulse) are used to
represent zeros (or space) pulses. The general rule
for AMI is: if a given mark pulse is of positive polarity,
then the very next mark pulse will be of negative po-
larity and vice versa. This alternating-polarity rela-
tionship exists between two consecutive mark pulses,
independent of the number of 'zeros' that may exist
between these two pulses. Figure 123 presents an il-
lustration of the AMI Line Code as would appear at
the TxPOS and TxNEG pins of the Framer, as well as
the output signal on the line.
Data 1 0 1 1 0 0 0 1 0 1 1 1 1 0 1 1 0 1 1 0 0 1 1 0 0 0 0 1
TxPOS
TxNEG
Line Signal
NOTE: One of the main reasons that the AMI Line Code
has been chosen for driving transformer-coupled media is
that this line code introduces no dc component, thereby
minimizing dc distortion in the line.
5.2.5.1.1.2 The HDB3 Line Code
The Transmit E3 Framer and the associated LIU IC
combine the data and timing information (originating
from the TxLineClk signal) into the line signal that is
transmitted to the remote receiver. The remote re-
ceiver has the task of recovering this data and timing
information from the incoming E3 data stream. Many
clock and data recovery schemes rely on the use of
Phase Locked Loop technology. Phase-Locked-Loop
(PLL) technology for clock recovery relies on transi-
tions in the line signal, in order to maintain lock with
the incoming E3 data stream. However, PLL-based
clock recovery scheme, are vulnerable to the occur-
rence of a long stream of consecutive zeros (e.g., the
absence of transitions). This scenario can cause the
PLL to lose lock with the incoming E3 data, thereby
causing the clock and data recovery process of the
receiver to fail. Therefore, some approach is needed
to insure that such a long string of consecutive zeros
can never happen. One such technique is HDB3 en-
coding. HDB3 (or High Density Bipolar - 3) is a form
of AMI line coding that implements the following rule.
In general the HDB3 line code behaves just like AMI
with the exception of the case when a long string of
consecutive zeros occur on the line. Any string of 4
consecutive zeros will be replaced with either a
"000V" or a "B00V" where "B" refers to a Bipolar
pulse (e.g., a pulse with a polarity that is compliant
with the AMI coding rule). And "V" refers to a Bipolar
Violation pulse (e.g., a pulse with a polarity that vio-
lates the alternating polarity scheme of AMI.) The de-
cision between inserting an "000V" or a "B00V" is
made to insure that an odd number of Bipolar (B)
pulses exist between any two Bipolar Violation (V)
pulses. Figure 124 presents a timing diagram that il-
lustrates examples of HDB3 encoding.
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